1. Field of the Invention
The present invention relates to a two-cycle engine, i.e. a two-stroke internal combustion engine (hereinafter called “two-stroke engine”), used on a portable power working machine.
2. Description of the Related Art
In a cylinder block of a two-stroke engine used on a portable power working machine such as chainsaws, brush cutters, and blowers, formed are an intake passage leading to a crank chamber, an exhaust passage leading to a combustion chamber of an upper portion of the cylinder, and a scavenging passage that makes the crank chamber and the combustion chamber communicate with each other.
In a two-stroke engine, an air-fuel mixture gas (hereinafter called “a mixture gas”) flows into the crank chamber through the intake passage. Then, the mixture gas flows into the combustion passage, and the mixture gas is combusted in the combustion chamber, and by an expansion power of the mixture gas when combusted in the combustion chamber, a piston is reciprocated in the cylinder (for example, see Japanese Unexamined Patent Application Publication No 2009-002311).
In the above-described two-stroke engine, when the piston descends after the combustion of the mixture gas, an exhaust port of the exhaust passage opens up to the upper portion of the cylinder, and the post-combustion gas in the combustion chamber is exhausted to the exhaust passage (exhaust process). When the piston descends further, a scavenging port of the scavenging passage is opened to the upper portion of the cylinder, and the mixture gas in the crank chamber flows into the combustion chamber through the scavenging passage (scavenging process).
Conventionally, in the scavenging process of the two-stroke engine, because both the exhaust port and the scavenging port are opened to the cylinder, the unburned mixture gas flowed into the combustion chamber from the scavenging port is also exhausted to the exhaust port together with the post-combustion gas in the combustion chamber. When the amount of unburned gas contained in the exhaust gas increases, the amount of hydrocarbon (HC) contained in the exhaust gas increases.
In the conventional two-stroke engine, when scavenging efficiency and combustion efficiency are low, there has been a problem in that the amount of carbon monoxide (CO) contained in the exhaust gas is increased.
The present invention has been developed to solve the above-described problem, and an object of the invention is to provide a two-stroke engine that allows the unburned gas contained in the exhaust gas to be reduced, and the scavenging efficiency and the combustion efficiency to be improved.